1. Field of the Invention
The present invention relates generally to a dampening device for a lithographic press. More specifically, the invention relates to a nozzle type dampening device which has a nozzle means ejecting a dampening liquid toward a roller.
2. Description of the Related Art
A lithography employs a substantially flat plate, in which hydrophobic property is provided for an image portion and hydrophilic property is provided for a non-image portion. On the surface of such plate, a water base dampening liquid and an oil base ink are supplied. Then, due to mutual repulsion between the dampening liquid and the ink, only ink can be supplied to the image portion. In order to supply the dampening liquid, a dampening device is employed. The dampening device has generally two kinds of mechanisms.
In one kind of the mechanism to be employed in the dampening device, a roller train from a dampening liquid storage portion to the plate surface is provided. By a peripheral surface of a rotating roller partly dipped within the dampening liquid in the dampening liquid storage portion, the dampening liquid is carried from the dampening liquid storage portion. The dampening liquid thus carried on the peripheral surface of roller is then transferred to the peripheral surface of the adjacent next roller which rotates with contacting with the former roller. Thus, the dampening liquid is transferred sequentially through the rollers. As a result, the dampening liquid is applied on the surface of the plate.
In this type of the dampening device, uniform thin layer of dampening liquid can be formed over the entire axial direction of the roller. However, it is difficult to locally vary supply amount of the dampening liquid per respective portions in the axial direction of the roller. Also, through the sequence of the roller train, the ink on the surface of the plate may be transferred to the dampening liquid storage portion to contaminate the dampening liquid.
Another type of the dampening device has been designed for providing a solution for the drawback in the former type of the dampening device. In this type of the dampening device, a supply source of the dampening liquid is separated from the surface of the plate or the roller train reaching the plate surface, and the dampening liquid is filed toward the plate surface or the roller train reaching the plate surface to enable differentiation of the supply amount of the dampening liquid per portions in the axial direction of the roller. Such type of the dampening device includes a nozzle type dampening device ejecting the dampening liquid from the nozzle as disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. Showa 51-59511, Heisei 1-110146, and Heisei 5-330009 and so forth.
The dampening device disclosed in Japanese Unexamined Patent Publication No. Showa 51-59511 is designed to supply respectively adjusted amounts of the dampening liquid to a plurality of nozzles and to supply air by means of a blower to spray the dampening liquid in atomized form by a high velocity air flow.
In the disclosed device, operation of the driving motor is controlled so that a metering pump may be driven at a desired speed corresponding to a speed of the press.
The dampening device disclosed in Japanese Unexamined Patent Publication No. Heisei 1-110146 includes a pump unit for supplying the dampening liquid, a nozzle ejecting the dampening liquid supplied from the pump unit, and a control unit for controlling ejection amount of the dampening liquid from the nozzle depending upon a printing speed of the press.
The ejection amount of the dampening liquid is controlled by adjusting a period to open each of the nozzles at an ejection timing in relation to rotation speed of a plate cylinder of the press, which ejection timing is determined on the basis of a preliminarily set and stored basic value, an adjusting value set by input depending upon the image to be printed with respect to each ejection nozzle and a correction value preliminarily set and stored relative to each printing speed of the press. Namely, in the disclosed device, control is performed so that given amount of ejection of the dampening liquid is effected at predetermined magnitude of angular displacement of the plate cylinder of the press.
It should be noted that, while there is no specific disclosure in Japanese Unexamined Patent Publication No. Heisei 1-110146, in addition to the control set forth above, the ejection amount of the dampening liquid can be controlled by adjusting the ejection period, i.e. a period to keep the nozzle open, or by adjusting the ejection pressure. In the alternative, the ejection amount of the dampening liquid may be controlled by adjusting an open area of the nozzle by means of a shutter member arranged in front of the nozzle.
Japanese Unexamined Patent Publication No. Heisei 5-330009 discloses the dampening device including a speed detecting means for detecting printing speed of the press, a memory storing supply amount of the dampening liquid to be supplied corresponding to a printing condition and the printing speed of the press, an ejection means connected to the dampening liquid source and an air source through piping for ejecting the dampening liquid in the atomized flow by the high velocity air flow toward the plate cylinder or onto the peripheral surface of the roller contacting with the plate surface, and a pressure control means provided within the piping connecting the dampening liquid source and the ejection means and performing control for the supply pressure of the dampening liquid on the basis of the supply amount of the dampening liquid stored in the memory.
In Japanese Unexamined Patent Publication No. Heisei 5-330009, there is a disclosure that supply amount of the dampening liquid is set corresponding to the printing condition, such as the printing speed, humidity, temperature and so forth, the supply amount of the dampening liquid thus set and the pressure value in the piping downstream of the pressure control means for regulating the pressure of the dampening liquid to be ejected at the downstream side of the pressure control means to control the ejection amount of the dampening liquid, in the shown dampening device. Furthermore, a needle valve is provided at the upstream side of the nozzle per each nozzle. The needle valve serves for fine adjustment of the supply amount of the dampening liquid. By fine adjustment, ability of constant amount supply of the dampening liquid can be improved.
On the other hand, it has been confirmed that the nozzle to be used in the nozzle type dampening device tends to slightly vary spreading angle of the dampening liquid ejected through the nozzle depending upon the supply pressure of the dampening liquid.
In general, the amount of fluid (dampening liquid) to be ejected through the nozzle is not uniform over the entire distribution area. It is typical that the amount of the dampening liquid becomes smaller in the vicinity of the circumferential portion of the distributing region. Accordingly, in the conventional nozzle type dampening device in the printing press, interval of the nozzles and distance between the nozzle and the target to which the dampening liquid is to be ejected are set so that the distributing regions of adjacent nozzles can slightly overlap in the width direction of the plate or in the axial direction of the roller so that the shorting amount of the dampening liquid at the peripheral portions of the distributing regions can be mutually compensated by overlap of the distributing regions.
However, through various printing tests utilizing such nozzle type dampening device, while the reason is not completely clear, it has been found that even though the supply amount of the dampening liquid is adjusted depending upon the printing condition, the amount of the dampening liquid becomes excessive at the portion of the printing surface corresponding to the overlapping portion of the distributing region of the dampening liquid.